This invention relates to a method of carburizing and quenching for steel members used in, for example, transmission gears of cars, which are required surface hard layers for wear-resistance and tough inner cores.
Conventionally, the method of carburizing and quenching has been used to meet the above-mentioned requirement of steel members used in transmission gears of cars. However, with the recent advent of engines of higher output and light, simplified transmissions, the need to improve a dedendum's resistance to bending fatigue failure and a tooth surface's resistance to pitting and fusion has arisen.
A conventional method of carburizing and quenching is carried out mainly to raise fatigue strength of steel members. An ideal structure of a heat-treated steel member is the mixture of martensite and some retained austenite. However, a surface hardness of an usual steel member composed of this mixed structure is at most no more than Hv800. To improve the pitting resistance and the fusion resistance of a steel member, the surface hardness of more than Hv800 is required. It is known that this requirement is met by precipitating carbide in the surface layer of a steel member.
As one of methods to meet this requirement, the carburizing and quenching method of reheating and quenching steel members after pre-carburizing and cooling has been proposed as disclosed in Japanese Pat. application Publication Gazette No. 62-24499.
This method of carburizing and quenching can precipitate carbide in the surface layer of a steel member. However, while enough carbide to achieve the surface layer hardness of more than Hv800 can be obtained, network carbide usually precipitates in the surface layer of the steel member. Network carbide precipitated in the surface layer causes stress concentration on the interface. This stress concentration leads to the reduced pitting resistance of the steel member.
Precipitation of network carbide does not occur if the cooling process after pre-carburizing is accelerated. But in this case, the amount of heat treatment deformation after carburizing and quenching with reheating a steel member becomes too large. Accordingly, the method of increasing the cooling speed after pre-carburizing is not desirable for preventing precipitation of network carbide.